Massive Multiple-Input Multiple-Output (mMIMO) enables space-division multiple-access (SDMA) if the channel matrix between transmit and receive antennas is known. Radio channel estimation is an essential part as it allows separating the data streams associated to different UEs.
To estimate the radio channel between different nodes, i.e. UEs, base station, access node, radio head, hyper transmitters etc., one generally transmits signals known at both transmit and receive nodes. These known signals are referred to as reference signals or pilot signals, which are also called reference symbols and pilot symbols, respectively. Using these pilot symbols one can estimate the unknown radio channel between transmit and receive nodes. Sending the pilot symbols in general leads to a loss in spectrally efficiency as it requires additional time-frequency resources to be used. The number and density of pilot symbols depend on the number of antennas and time-frequency characteristics of the radio channel. To acquire the radio channel in the time-frequency grid in the Time-Division Duplex (TDD) mode for mMIMO communication in a cell with K UEs each with a single antenna, K orthogonal pilot symbols, each associated to a UE, are required over a time-frequency grid of the size Tc×Bc where Tc denotes the coherence time and Bc denotes the coherence bandwidth of the channel. A coherence time Tc, is the number of symbols for which the channel approximately remains unchanged in time domain. A coherence bandwidth Bc is an approximate maximum bandwidth or frequency interval over which two frequencies of a signal are likely to experience comparable or correlated amplitude fading.
For TDD mMIMO communications when the number of antennas is very large the limiting factor for achieving high network throughput is the user mobility captured in the coherence time and bandwidth. FIG. 1 shows the behavior of the aggregate rate changes along with a number of active UE equipment (UE). The aggregate rate, also referred to as sum-rate, increases with UEs quantity up to a certain number and it then decreases. The optimal number of scheduled UEs in the conventional TDD is
      ⌊                            T          c                ⁢                  B          c                    2        ⌋    .The sum-rate increases by scheduling one additional UE as long as the number of UEs are less than
      ⌊                            T          c                ⁢                  B          c                    2        ⌋    .Scheduling bigger number of UEs, results to larger overhead in order to ensure orthogonal pilot transmission for channel estimation and hence the transmission is limited up to
  K  =            ⌊                        T          c                ⁢                  B          c                    ⌋        -    1  UEs. Scheduling additional UEs, it means that the pilot sequences for channel estimation should be reused, because there are no enough orthogonal pilot sequences when the number of UEs is above └TcBc┘. Therefore, the phenomena known as pilot contamination causes a severe degradation in the performance. Therefore the problem of conventional art is it cannot schedule more UEs beyond
  ⌊                    T        c            ⁢              B        c              2    ⌋in order to enable an enhanced aggregate rate, i.e. spectral efficiency of transmission.